This invention relates to an open circuit current transformer protection circuit for power supply systems and particularly to a triggered circuit connected across the secondary of a power line current transformer and responsive to transformer load loss or malfunction to short circuit and protect the current transformer.
In three-phase power supplies and the like, a current transformer is generally coupled to each alternating current line to provide an isolated output to an instrumentation or controls. The primary winding of the current transformer is connected in series with a phase line while the secondary winding is magnetically coupled to the primary winding by a suitable magnetic core. The secondary winding provides output power to a meter, a control or the like. It is well known that if the current transformer loses its load, the voltage in the secondary winding rises rapidly, being limited only by the saturation characteristic of the core. In the absence of any other limiting means, the voltage will rise above the maximum rated voltage of the current transformer insulation, the connectors and the like, with significant damage to the transformer. Further, the high voltage creates a relatively hazardous condition to personnel who may be working with the transformer. A voltage below that which will cause damage or hazard is typically selected as a critical level and protective means are often used to insure operation at or below the critical level.
Various protective circuits have been suggested. For example, U.S. Pat. No. 3,723,812 assigned to the same assignee as this invention, discloses a pair of voltage level responsive semi-conductor switch means, such as zener diodes connected back-to-back, to establish protection during each half cycle of the alternating current output of the current transformer. In the event of a load loss, the increasing abnormal voltage reaches the zener breakover voltage and the one zener diode conducts to establish a short circuit directly across the secondary winding preventing the voltage from rising to damaging and dangerous voltage levels. The impedance of the winding limits the current flow. Other systems have also been suggested, which use a bidirectional conducting thyristor, generically referred to as a triac. For example, the present assignee of this invention has built and made available short circuit protection on one half cycle using a triac connected in parallel with the secondary winding. A diac, which is a bidirectional breakdown diode, in series with a resistor connected to the gate provided a voltage sensitive circuit for triggering the triac when the voltage rose above a selected dangerous level. U.S. Pat. Nos. 4,075,510 and 3,727,104 disclose other triac circuits for producing shorting circuits for protecting a load or a switching device. U.S. Pat. No. 4,075,510 discloses an A.C. supply system including either a single triac or a pair of back-to-back triacs connected in series with a load. A switching circuit is connected to the gate or gates and selectively triggers the triacs to control the power level applied to the load. A metal oxide varistor may be connected to directly sense the voltage across the triacs, which connection may include connection to the gates or may be between the main triac electrodes. The triac(s) have a finite off-state breakdown voltage rating and the varistor serves as a simple voltage responsive device connected in parallel with the triac to ensure that transient voltages above the breakdown voltage do not occur. In this triac protective system, the varistor carries full current and repeated operation will significantly effect the life of the unit. Further, because of the characteristic of a varistor, a heavy current discharge must be carried by the varistor. The heavy current discharge will adversely effect the life of the varistor, particularly with repeated, continuous halfcycle triggering as in the case of load loss of a current transformer in contrast to an occasional transient fault. U.S. Pat. No. 3,727,104 discloses a constant current regulator having a triac connected in parallel with the regulator and the lamp to protect the source if the lamp load is lost. A gate circuit consisting of a capacitor in series with a resistor is connected in parallel with the Triac, with the common center node connected directly to the gate. A conventional capacitor coupling tends to make the gate circuit susceptible to other fast transient voltages which might interfere with proper control.
Thus, in monitoring and protective systems for power supplies and the like, the protective circuit must provide a reliable system capable of repeated carrying of high voltage and high current while maintaining temperature stability and having a long operating life. Although various prior art systems have been suggested, there is a need for a temperature stable and long life protective circuit.